We propose to study the connections of the cat primary auditory cortex (AI) with the inferior colliculus, medial geniculate body, and contralateral auditory cortex. Using anterograde transport methods to label terminal axonal fields and horseradish peroxidase to mark somata, the terminal zones and types of neurons contributing to these pathways will be defined. Particular types of morphologically characterized neurons can be assigned to particular functional roles, e.g., corticollicular, corticothalamic, or commissural. The goal is to clarify the anatomical relations of the midbrain, diencephalic, and telencephalic parts of the auditory system. This has important implications for cerebral localization and restitution of function, praticularly with regard to the auditory consequences of head injury, aging, or infectious agents. The sensory substrates for monaural and binaural phenomena, frequency analysis, and of speech must involve each of these pathways. Since connectional, structural, and neurochemical evidence suggests that not all parts of this pathway have the same role, this proposal concentrates on the primary pathway between thye central nucleus of the inferior colliculus, the ventral nucleus of the medial geniculate body, and AI, and the reciprocal circuits often interconnecting them. We will define the cells of origin, their topographical projections, and their covergence or divergence. Our results have implications for sreial and hierarchical models of information processing. Mixtures of tracers will be injected into physiologically defined areas in the auditory midbrain, thalamus, or cortex. A study in the rat will examine the inferior colliculus neurons projecting to the ipsilateral medial geniculate body or to the contralateral inferior colliculus, and the distribution of their axon terminals. We shall also study the auditory cortical neurons accumulating the putative inhibitory neurotransmitter, gamma-aminobutyric acid, and glutamic acid decarbosylase. Finally, the structure of layers V and VI neurons will be analyzed. These connectional, transmitter-specific, and structural inquiries form a more secure basis for interspecific comparisons with regard to neural homology, and for deriving the circuits of auditory neorcortes. These observations will provide a framework for comparing the organization of AI with the primary somatic sensory and visual cortices.